1. Field of the Invention
This invention relates to respiratory systems useful in supplying oxygen to neonates, and more particularly to apparatus for measuring oxygen uptake in such systems.
2. Prior Art
In situations where premature or term infants less than one year of age require ventilatory support, special problems occur unique from the conventional systems used for adult respiration. Since the breath volume changes are much smaller in neonatal systems (0.7-2.7 liters per minute), previous methods for controlling system gain response have not been easily adaptable. Furthermore, the high respiratory rate (20-60 per minute) and low tidal volume (7-10 ml/kg) of neonates create additional problems in design for a neonatal respirator with oxygen consumption monitoring capability.
Further limitations rise with respect to those infants having Respiratory Distress Syndrome (RDS) inasmuch as such infants must always be kept under Continuous Positive Airway Pressure (CPAP), usually in a range of 3-5 mm Hg. CPAP is necessary in such cases to increase functional residual capacity, to improve compliance of the lung, and to prevent atelectasis. In severe RDS and RDS with pneumonia, the neonate is generally kept on CPAP and also on Intermittant Mandatory Ventilation (IMV). In addition to meeting the aforementioned requirements, an effective neonatal ventilation system must also provide a high flow of oxygen enriched air (5-15 liters per minute) directed past the endotracheal tube of the infant so that he always inspires fresh air.
FIG. 1 illustrates a positive-end-expiratory-pressure respirator which has the capability to incorporate CPAP and CPAP-IMV modes and to provide a high flow of oxygen enriched air through a respirator loop. No provision is made, however, for separate retrieval of expired air from the neonate so that oxygen consumption can be effectively monitored. Instead, both inspiration and expiration occur through the same tube opening 20 to the neonate with the expired volume of air re-entering the primary flow line of the high flow oxygen circuit. The eventual return of expired air to the flow line is essential in such units in order to preserve a constant positive pressure throughout the ventilatory system. Therefore, because of an intermittent addition of small volumes of expired air to the continuous high rate of flow and high oxygen content in this primary flow line, a comparison of oxygen content before and after inspiration has not been practical.
Numerous benefits could be realized by a ventilatory system which permits measurement of oxygen consumption or uptake by neonates. Metabolic rate, for example, can be usefully monitored since it directly relates with oxygen consumption. Previously, metabolic rate has been determined by means of skin temperature probes which detect the environmental temperature within the incubator. The obvious time lag and potential inaccuracy of this method suggest that a substantial benefit would arise with a technique of direct measurement of oxygen consumption. An additional need for measurement of oxygen consumption arises with infants having a pulmonary disease who must be ventilated with an increased inspired oxygen fraction F.sub.IO.sbsb.2. Too high a F.sub.IO.sbsb.2 can be highly toxic and damaging to the lungs. By monitoring the extent of oxygen uptake, the value for F.sub.IO.sbsb.2 can be reduced to an optimum level, without endangering the infant.
Infants having RDS which are ventilated under CPAP may experience a condition referred to as alveolar recruitment, as well as the aforementioned increase in lung compliance and functional residual capacity. At present, the CPAP settings on a respiratory system for neonates are arbitrary and must be obtained by trial and error. If the CPAP is set at high levels, it may cause alveolar overdistension or may even decrease the compliance of the lungs and also decrease the alveolar ventilation, so that the infant breathes harder in order to maintain alveolar ventilation. By monitoring oxygen consumption at different CPAP levels, the optimal airway pressure can be determined to avoid the aforementioned adverse side effects.
Finally, infants with severe RDS, RDS with pneumonia, bronchopulmonary dysplasia, etc., have to be managed with CPAP and Intermittent Mandatory Ventilation (IMV). The selection of optimal IMV settings on the respirator is very difficult since there are many parameters to be taken into account, such as air way pressure, inspiratory-expratory rate, IMV and CPAP. At present, the CPAP-IMV settings are obtained through trial and error by blood gas analysis on the infant. Monitoring of oxygen consumption would provide a more direct analysis for the optimal conditions and settings.